586  
FXUS02 KWBC 091600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT THU AUG 09 2018  
 
VALID 12Z SUN AUG 12 2018 - 12Z THU AUG 16 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE MEDIUM RANGE PERIOD FEATURES A WELL FORECASTED UPPER CLOSED  
LOW TEMPORARILY MEANDERING NEAR THE OHIO VALLEY SUN-TUE BEFORE  
GETTING KICKED NORTHEASTWARD BY CANADIAN TROUGHING LATE IN THE  
WEEK. A SEPARATED SOUTHWESTWARD EXTENSION OF THE OHIO VALLEY LOW  
WILL ALSO LINGER ACROSS THE SOUTHERN PLAINS REGION LEADING TO A  
HEAVY RAIN THREAT FOR TEXAS/OKLAHOMA THIS WEEKEND AND INTO EARLY  
NEXT WEEK. OUT WEST, STRONG RIDGING SHOULD HOLD ACROSS THE  
SOUTHWEST, WHILE POSSIBLE WEAK SHORTWAVES ROUND THE TOP OF THE  
RIDGE ACROSS THE PACIFIC NORTHWEST AND NORTHERN INTERMOUNTAIN WEST.  
 
THE LATEST MODELS AND ENSEMBLES SHOW GOOD CLUSTERING THROUGH DAY 5  
FOR MUCH OF THE CONUS. BY DAYS 6-7, NORMAL RUN-TO-RUN  
DISCONTINUITIES AND TIMING/INTENSITY DIFFERENCES ARISE,  
PARTICULARLY WITH THE OPERATIONAL MODELS. THE ONE THING TO NOTE IS  
THE 06Z GFS LOOKED QUITE DIFFERENT FROM ITS PREVIOUS RUNS LATE IN  
THE PERIOD, PARTICULARLY OUT WEST, AS IT MAINTAINED STRONG RIDING  
WELL INTO THE INTERMOUNTAIN WEST. ITS PREVIOUS RUNS ALONG WITH THE  
ECMWF, AND TO SOME EXTEND THE GEFS/EC MEAN ENSEMBLES (ALBEIT  
SOMEWHAT WASHED OUT) INDICATE POSSIBLE WEAK SHORTWAVE RIDING  
OVERTOP THE RIDGE. FOR THIS REASON, OPTED TO USE THE 00Z RUN OF  
THE GFS RATHER THAN THE 06Z RUN TO MAINTAIN A SOLUTION CLOSER TO  
CONSENSUS, AS WELL AS CONTINUITY WITH SHOWING SOME SEMBLANCE OF  
WEAKNESS IN THE UPPER RIDGE ACROSS THE WEST. THIS CYCLE OF THE  
MEDR PROGS USED A BLEND OF THE WELL CLUSTERED 00Z ECMWF/GFS WITH  
THEIR RESPECTIVE ENSEMBLE MEANS.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
A RELIEF TO THE WESTERN U.S. HEAT IS FINALLY IN SIGHT THIS WEEKEND  
ACROSS THE NORTHWEST AND HIGH PLAINS SUNDAY/MONDAY AS UPPER  
TROUGHING AND A SURFACE COLD FRONT MOVE THROUGH. THE UPPER  
TROUGH/LOW OVER THE SOUTHERN PLAINS WILL HELP FUEL ORGANIZED RAIN  
AND STORMS ACROSS TEXAS AND WESTWARD INTO THE SOUTHWEST/FOUR  
CORNERS REGION, PARTICULARLY ALONG THE FAVORED TERRAIN. GUIDANCE  
CONTINUES TO WAVER ON EXACT PLACEMENT, BUT SEVERAL INCHES OF RAIN  
ARE LIKELY NEAR THE RED RIVER VALLEY (WEST CENTRAL TX INTO OK)  
DURING THE SUNDAY TO THURSDAY TIME FRAME. RAINFALL LIFTS INTO THE  
MISSISSIPPI VALLEY AND THE OHIO VALLEY LATE IN THE WEEK AHEAD OF  
UPPER TROUGHING ACROSS THE REGION LATER IN THE WEEK. MONSOONAL  
MOISTURE AND POSSIBLE LOCALLY HEAVY RAINFALL WILL CONTINUE ACROSS  
THE SOUTHWEST.  
 
A WAVY FRONT WILL LINGER ACROSS THE U.S. SOUTHERN TIER DURING THE  
PERIOD WHICH WILL HELP SPARK AFTERNOON SHOWERS/STORMS AND LOCALLY  
HEAVY RAINFALL WITHIN THE WARM AND HUMID AIRMASS. AS THE OHIO  
VALLEY UPPER LOW SLOWLY LIFTS TOWARD NEW ENGLAND, MORE WIDESPREAD  
PRECIPITATION MAY BE FAVORED TO ITS EAST SIDE ALONG THE  
APPALACHIANS TO THE I-95 CORRIDOR WHERE THE GROUND REMAINS QUITE  
SATURATED IN SOME PLACES. THIS COULD DEVELOP INTO A HEAVY RAINFALL  
THREAT CONSIDERING THE RELATIVELY SLOW SYSTEM TRANSLATION AND  
SOUTHERLY FETCH OF AMPLE MOISTURE OVER THE COURSE OF SEVERAL DAYS.  
SUMMERTIME CLOSED LOWS CAN BE UNDERESTIMATED IN TERMS OF  
PRECIPITATION INTENSITY/AMOUNTS AND LONGEVITY.  
 
ANOTHER FRONTAL BOUNDARY WILL DROP OUT OF CANADA TOWARD MONTANA  
NEXT THURSDAY OR SO WITH A RETURN OF 90+ DEGREE TEMPERATURES  
ACROSS THE HIGH PLAINS.  
 
SANTORELLI/FRACASSO  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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